The high frequency compression speaker driver of the invention relates generally to audio speakers intended to reproduce high frequency sounds at high acoustic output levels, and specifically to the diaphragms and their mounting structures used in high frequency compression drivers.
A compression driver consists of a light circular diaphragm having a spherical dome section suspended from a mounting plate. Firmly attached to the edge of the diaphragm's dome is a cylindrical former and wound on this is a coil of wire, the voice-coil. When a signal is applied to the coil, a force is exerted which causes the dome to move. The movement of the dome, responding to the variations in the electrical signal, sets up sound waves in the air.
To produce high frequencies, movement of the diaphragm must be in as straight a path axially as possible, with a minimum of flexing or distortion of the diaphragm itself, and a minimum of side-to-side (radial) movement. Moreover, a diaphragm's output will naturally decrease at high frequencies due to its mass, i.e., mass induced high frequency roll-off.
In all compression drivers the dome is attached to a mounting ring or base via a compliant material known as the "diaphragm compliance." The "compliance" allows the dome to move up and down in response to the electrical signal fed to the coil and centers the dome both vertically and horizontally.
An important aspect of the diaphragm's performance at high frequency is its dome resonance frequency. The dome resonance frequency is the dome's natural frequency of vibration. If the diaphragm is driven at this resonance frequency it will produce a greater output than it will if it is driven at a somewhat higher or a somewhat lower frequency. Therefore the resonance frequency can be utilized to partially offset the mass induced high frequency roll-off and thereby extend the useful range of a compression driver.
Resonance frequency is dependent upon the material and curvature of the dome. Thus, this frequency can easily be calculated from the properties of the material and the radius and length of the spherical section. Some of the materials used for construction of the diaphragm in high frequency compression drivers include aluminum, beryllium, and titanium. Heat treatable aluminum is a particularly good compromise for the diaphragm, as it is light-weight, relatively stiff, has a high fatigue strength, and has a high damping tendency that turns part of the unavoidable distortion of the moving diaphragm into heat, rather than into distorted sound.
As the use of audio speakers to reproduce music has increased in recent years, the need has arisen for drivers to reproduce frequencies up to 20 kHz with minimal distortion, while withstanding the power output of large amplifiers connected to electrical musical instruments. To the design engineer, the requirements for high frequency response coupled with high power handling present formidable challenges. High frequency performance requires light, low mass coils and diaphragms. On the other hand, high power handling is better served by substantial coils and diaphragms which, however, because of their very mass are inefficient at higher frequencies.
Because of this dilemma, the mid to high frequency range is usually divided into two bands and covered by physically different driver units. The lower end (mid-range) is serviced by drivers with relatively heavy diaphragm assemblies, the high end is covered by drivers equipped with light diaphragms and small diameter coils. Several of the smaller drivers are then required to match the output of each of the large mid-range units. The solution is reliable, but not altogether satisfactory, because of the obvious penalties in cost, size and weight.
In the past, efforts have been made to couple high frequency response with high power handling in a single compression driver unit. The assignee of the present application, Renkus-Heinz, Inc. in its drivers bearing model numbers 1400, 1800, and 3300, attaches a silicon compliance ring around the perimeter of the aluminum dome and to the mounting plate. While having the advantage of being resistant to fatigue, the silicon compliance is not restrictive in the radial direction of the diaphragm, and thus has little influence on the resonance frequency of the dome. Since the compliance does not significantly affect the resonance frequency, the compliance cannot be used to adjust the resonance frequency to a more desired position in the frequency spectrum to increase the high frequency performance of the diaphragm. The resonance frequency is determined substantially only by the properties of the dome material and the curvature chosen for the dome.
Pioneer Electronics Corporation of Japan offers compression drivers that are constructed similarly to the Renkus-Heinz drivers just described, with a dome of a light, stiff material held by a soft suspension or compliance. But, rather than using aluminum for the dome, the Pioneer drivers have used beryllium for the dome. Beryllium is lighter and stiffer than aluminum; thus the moving mass of the diaphragm is reduced and the resonance frequency of the dome is increased which extends the high frequency performance of the driver. But, the cost of manufacturing these drivers with beryllium domes is quite high. Also, beryllium is quite brittle and shatters easily on application of a large amount of power. This tendency to shatter makes the beryllium domes unreliable for high power applications. Also, the beryllium has a high "Q" factor, which means that distortions in the dome are not damped and converted to heat, and thus the dome distortion produces high audible distortion. Further amplification of the Pioneer drivers is found in "The Influence of Parasitic Resonance on compression driver loudspeaker performance" by Shozo Kinoshita et al. preprint No. 1422(M-2) by the Audio Engineering Society 1978 and "Design of 48 MM Beryllium Diaphragm Compression Driver" by Shozo Kinoshota et al., preprint No. 1364 (D-9) by the Audio Engineering Society 1978 which are herein incorporated by reference.
James B. Lansing Sound, Inc. (JBL) of Northridge, Calif. has constructed drivers using aluminum for the dome, and forming with the diaphragm dome an annular compliance of aluminum consisting of a series of pyramid-like structures that give the compliance flexibility in the axial direction, but stiffness in the radial direction. The radial stiffness of the compliance produces a higher resonance frequency for the diaphragm, and improves the high frequency performance of the device. The JBL device is disclosed in U.S. Pat. No. 4,324,312 to Durbin. But, the corner at which the dome portion of the diaphragm merges into the compliance portion is sharp, with the result that most flexing of the compliance occurs at this corner. The eventual fatiguing of the aluminum causes the dome-to-compliance boundary to fail. To solve this reliability problem, JBL has switched to using titanium for the dome and compliance combination of the diaphragm. But, titanium is much heavier than aluminum, which increases the mass of the diaphragm and consequently worsens the high frequency performance of the diaphragm. Tilanium also has relatively high "Q" and therefore produces audible distortion.
There is, therefore, a need for single compression units which economically combine high frequency performance with high power handling using materials that result in low overall distortion.